I. Field of the Invention
This invention relates to drilling tools and more particularly to helically fluted twist drills.
Helically fluted twist drills are the most commonly used drilling tools and are generally required to perform severe machining operations under extremely adverse conditions. The cutting end of a helically fluted twist drill includes a pair of cutting lips on opposite surfaces of an intermediate web, the width of which is typically 12 to 20 percent of the diameter. The chisel edge of the twist drill extends obliquely across the center of the web. The foregoing drills are typically long and slender and the helical flutes constitute a column eccentricity that tends to reduce the rigidity of the drill under axial thrust loads.
Oppositely directed cutting surfaces positioned at the cutting end of the shaft of the drill are subjected to axial and torsional loads which create material demands on the fabrication of the drill tool. Obviously the material of the cutting lips should be as hard as possible to cut a work piece and it should be heat resistant as well to maintain the cutting edge of the drill at elevated temperatures. Moreover the material of the body of the drill shaft must be both rigid and tough to resist deflection and to maintain the integrity of the drill under loads while the drill is being used. The foregoing requirements have resulted in compromises in material selection since hard materials tend to be brittle while tough materials tend to wear quite easily.
II. Description of the Prior Art
The prior art teaches a combination of materials having the characteristics of hardness and wear-resistance at the cutting surfaces and toughness and rigidity of the body and shaft. It has been previously proposed to form the cutting surfaces of one material and the body and shaft of another. This has resulted in a variety of combinations such as tungsten carbide or diamond inserts or tips on carbon steel or carbide shafts. These combinations while individually useful have a common disadvantage, i.e. the braze connection between the insert or tip and a shaft. Tungsten carbide can be soldered or brazed directly to the steel or carbide shaft. However a diamond tip or insert must first be adhered to a carbide substrate which is in turn soldered or brazed to the shaft. Diamond particles are typically formed into a compact or PCD (polycrystalline diamond disc) and bonded to a carbide substrate with a metallic catalyst in a high pressure-high temperature press. At atmospheric pressures however the metal which catalyzes the bonding of the diamond particles to each other and to the substrate in the press will also catalyze the conversion of diamond to graphite at temperatures above 700 degrees centigrade which will cause disintegration of the PCD compact. Accordingly a low temperature solder or braze connection is used to attach the substrate to the shaft. The aforementioned diamond discs as well as the diamond insert stud blanks for example are fabricated from a tungsten carbide substrate with a diamond layer sintered to a face of a substrate the diamond layer being composed of polycrystalline material.
The synthetic polycrystalline diamond layer is manufactured by Megadiamond Industries, Inc., a wholly owned division of Smith International, Provo, Utah. Another source of polycrystalline diamond is manufactured by the specialty material department of General Electric Company of Worthington, Ohio. The foregoing drill cutter blank is known by the trademark name of Stratapax Drill Blank.
Two examples of patents assigned to Megadiamond describe cutting elements for drilling holes. U.S. Pat. No. 4,527,643 teaches a cutting element for drilling holes which consists of five cutting edges which are comprised of polycrystalline diamond or the like mounted to a central carbide substrate of similar hard material held by a rotatable shaft which can be inserted into a drilling machine. The polycrystalline material is then supported with respect to torsional forces exerted upon it during drilling.
U.S. Pat. No. 4,627,503 describes a polycrystalline diamond and metal element for use as a cutting element for drilling holes or similar uses. The cutting element comprises a polycrystalline diamond center portion sandwiched between metal. The metal side portion is made from a soft metal having a Young's Modulus less than approximately 45 times 10 to the sixth power and is selected from a group comprising cobalt, nickel, iron, copper, silver, gold, platinum, palladium and alloys of these metals in metallic compounds containing these metals.
Both of these recently issued patents utilize a braze type bonding element to secure the diamond cutters within a drill blank. Typically a low temperature solder or braze connection is used to attach the substrate to a shaft such as the shaft of a helical twist drill. This braze connection limits the effective life of such drilling tools since it is softer than either the substrate or the shaft. The braze thus becomes the weakest point of the tool construction and the limiting factor in the tool usage.
U.S. Pat. No. 4,762,445 teaches a helically fluted twist drill apparatus in which offset opposed veins of sintered abrasive particulate, such as diamond, are embedded within a drill blank made of a less abrasive material such as carbide. The non-aligned veins of abrasive material, themselves intersect through juxtapositioning adjacent the point and web of the drill. The veins of diamond are 180 degrees opposed across the tip of the helical drill blank. The opposing veins intersect at the center or axis of the helical drill to provide a concentration of diamond at the tip of the twist drill.
A disadvantage of the foregoing patent is that the veins of diamonds are relatively shallow at the tip of the twist drill and tend to wear out rather quickly.
The present invention overcomes the problems of the foregoing prior art by providing, for example, a concentration of diamond in each of a pair of flutes of a twist drill. The two helical grooves join through a hole drilled through the center of the blank so that diamond is continuous, i.e.; in one groove, through the center to the opposite groove thereby providing a continuous layer of diamond across the cutting end of the twist drill.